The coating of semiconductor wafers is done in a spin coating machine by covering the top surface of a wafer with liquid photoresist while the wafer is spun slowly, then shutting off the supply of photoresist and spinning the wafer at high speed to remove the excess photoresist, thereby achieving a thinner, more uniform, coating. This coating can be applied through a nozzle supported by a fixed or moveable arm.
At the end of this process, a rim or "bead" of photoresist remains on the edge of the wafer. It is desirable to remove this bead, as it is thought to be a source of loose particles, as a result of handling the wafers during processing, which may produce irregularities and defects in the semiconductor components on the wafer. Currently, as described in U.S. Pat. No. 4,518,678,this bead is removed by slowly spinning the wafer and applying solvent to the bottom of the wafer, from a solvent nozzle located under the wafer, thus causing some of the solvent to "wick" around the edge and wash off the photoresist bead on the top outside edge of the wafer. The solvent used is typically a special solvent for top edge removal.
The main disadvantage of this procedure is that the flow of solvent over the top of the wafer is uncontrolled, with the potential to damage some semiconductor devices thereupon as well. Using a top solvent nozzle 1 to apply a stream of solvent 2 to the top of a wafer 3, as shown in FIG. 1, in conjunction with the bottom solvent nozzle, can control the amount of solvent moving towards the inside of the wafer. However, this solution introduces its own inherent problem since the top solvent stream is applied tangentially to the wafer at an angle of about 45.degree. to 50.degree., and the top nozzle is therefore positioned over the wafer. When the solvent stream is shut off, there is a high probability that a drop 4 of solvent will remain hanging from the edge of the top nozzle, as shown in FIG. 2a, or a drop may free fall onto the wafer, as shown in FIG. 2b. This problem can only be partly overcome by the application of a slight negative pressure to the liquid in the solvent nozzle. This process, called "suck-back", is illustrated in FIG. 3, which shows that a drop 6 may remain at the nozzle's end. This drop may later fall onto the wafer after the process has been completed. This represents an unacceptable risk.